1. Field of the Invention
The present invention relates to a developing apparatus using monocomponent magnetic developer for developing electrostatic latent images.
2. Description of the Related Art
The basic constitution of a typical developing apparatus using monocomponent developer is disclosed illustratively in Japanese Patent Laid-open No. Sho 61-223769 filed by the same applicant having filed this invention. The disclosed apparatus utilizes a semiconductive phenolic resin as the material of the sleeve for a developing roller incorporated therein.
The roller sleeve made of the semiconductive phenolic resin allowed the above apparatus to offer a distinctive advantage over the apparatuses preceding it and operating on various non-contact developing principles. That is, in recording images, the apparatus reproduced black areas more clearly, graduated shadow images more distinctly and line images more sharply than ever before.
Since then, the need has been recognized for an apparatus capable of higher image quality, i.e., an apparatus having the ability to ensure higher levels of shadow and line image reproducibility without degrading the current level of black area reproducibility. Specifically, what has been required is the capability of copying line images faithfully, i.e., the ability to minimize both toner blurs in the vicinity of characters and the gradual thickening of lines observed when one copy reproduces another copy which in turn reproduces another, and so on.
The disclosed apparatus above fails to meet the new requirements. The reason is the limitation imposed by the electric resistance value of the developing roller sleeve. Although the electric resistance value is supposed to range from 10.sup.6 to 10.sup.12 .OMEGA..multidot.cm under prior art constraints, the controllable limit of the resistance value in effect when the above apparatus was proposed was 10.sup.9 .OMEGA..multidot.cm. Given that resistance value of the sleeve, machine designers had to restrict the other parameters primarily to meet the need for stable manufacture of developing rollers. Although a lower gradient .gamma. was desired of the image density characteristic curve with respect to the electrostatic image potential for image development, the high sleeve resistance value made it impossible to attain that objective.
A solution to the above problem is proposed in Japanese Patent Laid-open No. Sho 63-146064 having recourse to a device for addressing characteristics other than the electric resistance value of the sleeve. The proposed device takes note of the waveform of an alternating electric field applied both to a developing roller and to an image carrying member. What the proposed device does is to vary the relationship between two rise velocities: the velocity at which the alternating electric field is applied in the direction of promoting development (from the developing roller to the image carrying member), and the velocity at which the alternating electric field is applied in the direction of suppressing development (from the image carrying member to the developing roller).
In the above setup, however, the sleeve is not composed of the semiconductive phenolic resin but made of metal (AI, SUS, etc.). The device thus fails to resolve the problem sufficiently, due primarily to a reduced level of resistance to leaks (i.e., a wide gap between the developing roller and the image carrying member, coupled with a low peak-to-peak level of the alternating electric field).
Another solution to the above problem is proposed in Japanese Patent Laid-open No. Hei 4-97177. The proposed apparatus embodying the laid-open invention has the sleeve made of a semiconductive phenolic resin and is set to meet the condition EQU ta/T.ltoreq.2/5
where, T represents the waveform period of the alternating electric field, and ta denotes the rise time in effect when the alternating electric field is applied in the direction of promoting development (from the developing roller to the image carrying member). The newly proposed apparatus is also arranged to meet the condition EQU 2.5 g+750.ltoreq.VP-p.ltoreq.5 g+2000
where, g stands for the gap between the developing roller and the image carrying member, and Vp-p is the peak-to-peak value of the alternating electric field. With these features, the apparatus of the latter laid-open invention has contributed to reducing both the toner blurs in the proximity of characters and the growing thickness of lines observed in copy-to-copy reproduction.
The inventors of this invention examined the techniques of the laid-open patents and came up with new findings representing improvements for still higher image quality and for minimizing irregularities induced by the sleeve of the developing roller. These findings are incorporated in the present invention which is based primarily on the constitutions described in Japanese Patents Laid-open Nos. Sho 61-223769 and Hei 4-97177.